Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas on an anode electrode with an oxidant gas on a cathode electrode. Conventionally, the oxidant gas is oxygen or air, and in high temperature solid oxide fuel cells (SOFCs), operated at approximately 600° C. to 1000° C., the fuel gas is hydrogen or a mixture of hydrogen, carbon monoxide, and/or traces of hydrocarbons. The fuel gas may also contain non-fuel gases including nitrogen, water vapor and carbon dioxide. Yttria stabilized zirconia (YSZ) is typically used as a SOFC electrolyte because of its properties of heat resistance, high ionic conductivity, and low electronic conductivity. The YSZ electrolyte may be fabricated as a freestanding ceramic sheet or a ceramic coating on a substrate. A three-layer structure with a porous anode electrode on one side of the YSZ electrolyte and a porous cathode electrode on the other side of the YSZ electrolyte forms a complete electrochemical cell. A typical anode electrode is a cermet containing YSZ and nickel or copper, and a typical cathode electrode is lanthanum strontium manganite (LSM).
Each of the fuel cells described above produces a potential of less than about 1 volt, so multiple cells must be connected in series to produce a higher, more useful voltage. The series interconnection may be accomplished by constructing a bipolar stack of planar cells such that current flows from the anode of one cell to the cathode of the next cell. The stack output current is collected from the top and bottom cells at a voltage equal to the sum of the voltages of the individual cells. Fuel gas and oxidant gas must be supplied to each cell in the stack, while the fuel and oxidant gases are kept separate so that they only react with each other indirectly though the electrochemical fuel cell to generate electric current. Direct reaction of the fuel and oxidant gases can reduce energy conversion efficiency, and may generate high temperatures that damage the cell or stack structures. Barrier structures, seals, and flow conduits that separate the fuel gas from the oxidant gas are necessary elements in planar fuel cell stack assemblies.
U.S. Pat. No. 4,950,562 to Yoshida et al. relates to an exemplary prior art SOFC fuel cell stack assembly with external manifolds. The electrochemical cell is a thin rectangular structure including a solid electrolyte covered on one side by porous anode material and covered on the opposite side by porous cathode material. The cells are stacked together with rectangular bipolar separators that connect the anode of each cell in a stack to the cathode of an adjacent cell. Fuel gas flows through parallel grooves on the anode-contacting side that extend from a first edge of the rectangular bipolar separator to the opposite edge. Similarly, oxidant gas flows through parallel grooves on the cathode-contacting side that extend from a second edge of the rectangular bipolar separator to the opposite edge, such that the fuel and oxidant grooves are perpendicular to each other. Each of the four sides of the stack presents an array of openings for use as: fuel-in, fuel-out, oxidant-in, and oxidant-out. Corresponding fuel-in, fuel-out, oxidant-in, and oxidant-out manifold ducts cover the four cell stack sides and communicate with the arrays of openings. Current is collected from external terminals at the ends of the stack at a voltage equal to the total of the individual cell voltages. The bipolar separators and external terminals in Yoshida are coated with chromium-containing metal alloy that carries the current from one cell to the next cell, while resisting the effects of the fuel gas on the anode side and the oxidant gas on the cathode side at elevated operating temperatures. In addition, the alloy is selected to have thermal expansion characteristics compatible with the other components. Seals between the cells and the bipolar separators and external terminals are formed by glass paste. The four manifold ducts are formed by a ceramic tube slipped over the rectangular stack, with ceramic paste and glass paste seals between the stack corners and the inner diameter of the ceramic tube. In combination, these seals prevent mixing and direct reaction between the fuel gas and oxidant gas streams. However, external manifold cells have proven difficult in practice, in large part because of the difficulty sealing the manifolds to the cell stack. The seals must bridge irregularities in the stack edges, allow relative movements with temperature changes, and be electrically insulating to avoid shorting the cells. Due to at least these drawbacks associated with external manifolds, the use of internal manifolds is preferred.
U.S. Patent Application Publication 2002/0048699 to Steele et al. relates to an exemplary prior art SOFC cell stack assembly with internal manifolds. The electrochemical cell is formed as three layers on a porous chromium-containing stainless steel sheet that includes a non-porous border extending beyond the electrochemical cell. The first layer of the electrochemical cell is a porous cathode, followed by a dense electrolyte layer, and a porous anode layer. The cell is welded or brazed at the center of a rectangular metal bipolar separator that is larger than the cell, forming a cell module. Four apertures are formed through the bipolar separator in the border such that each aperture is positioned between an outer edge of the cell and an inner edge of the bipolar separator. Fuel gas flows through grooves on the anode-contacting side that extend from a first aperture in the bipolar separator to a second aperture near the opposite edge. Similarly, oxidant gas flows through grooves on the cathode-contacting side that extend from a third aperture in the bipolar separator to a fourth aperture near the opposite edge, such that the fuel and oxidant grooves are perpendicular to each other. The apertures align and form axial ducts when the cell modules are stacked, forming fuel-in, fuel-out, oxidant-in, and oxidant-out internal manifolds that communicate with the corresponding flow grooves. Current is collected from external terminals at the ends of the stack at a voltage equal to the total of the individual cell voltages. The bipolar separators in Steele are machined or stamped from chromium-containing metal alloy that carries the current from one cell to the next, while resisting the effects of the fuel gas on the anode side and the oxidant gas on the cathode side at elevated operating temperatures. In addition, the alloy is selected to have thermal expansion characteristics compatible with the other components. A compressible electrically insulating seal gasket is provided between the stacked cell modules. The gasket seals around the stack perimeter and between the fuel-in, fuel-out, oxidant-in, and oxidant-out internal manifolds to prevent mixing and direct reaction between the fuel gas and oxidant gas streams. Such a compliant seal is possible because of the maximum operating temperature of 500° C. of the particular electrochemical cell material system used in Steele.
Formation of fuel and oxidant gas flow grooves in metallic bipolar separators as described, e.g., in Yoshida and Steele requires the use of thick separators, but it would be cost-prohibitive to construct the separators from expensive materials such as noble metals. Further, such thick bipolar separators are rigid, and must have thermal expansion characteristics closely matched to those of the electrochemical cells to prevent excessive mechanical stress. The lack of compliance of the separators also requires use of a sealing means such as a compliant gasket or glass paste. As described above, the electrically insulating compliant gaskets used in Steele are limited to relatively low temperature systems.
While substitution of compliant metallic gaskets might raise the temperature limit to accommodate more typical SOFC material systems, the metallic gaskets would electrically short circuit the cells. Glass-based seal gaskets are described, e.g., in U.S. Pat. No. 5,453,331 to Bloom et al. and U.S. Pat. No. 6,271,158 to Xue et al. The glass and filler materials of the seal gaskets are selected such that the seal is somewhat viscous and compliant at the cell operating temperature, thereby adjusting to fill the gaps. However, the following drawbacks are apparent from this approach. The seals transition to elastic solids as the cell and stack assembly cools, which may generate significant stresses unless the solid glass is a good thermal expansion match with the cell and stack components. In addition, glass often wets the cell and stack materials, which can migrate from their original locations. Further, amorphous glass may crystallize at operating temperatures, changing its thermal and mechanical properties.
U.S. Pat. No. 4,997,727 to Bossel describes deep-drawn thin metal bipolar separators made of a chromium-containing superalloy. The separators have an egg carton-like shape that contacts and electrically connects adjacent electrochemical fuel cells while providing the required fuel and oxidant passages. The shape is also intended to provide a certain compressibility to maintain electrical contact. The electrochemical fuel cells include thick edges formed as an extension of the thinner active YSZ electrolyte sheet. These thick cell edges are shaped such that they interlock with the bipolar separators to form edge seals, eliminating the need for other sealing materials such as glass. This interlocking seal may prove difficult in practice since precise control of the planar dimensions of the bipolar separator and the electrochemical cell is required. Such dimensional control is particularly difficult for ceramic components such as the YSZ electrolyte that shrink during firing. The invention described in Bossel is configured for use with external manifolds, where internal manifold stacks are not disclosed.
U.S. Pat. Nos. 6,106,967 and 6,326,096, both to Virkar et al., describe electrochemical cells incorporating flow grooves, and a combined bipolar separator and sealing gasket made of flat metallic foil. The structural base of the electrochemical fuel cells is a die-formed, generally rectangular plate made of porous anode material. The fuel-contacting side has molded parallel fuel gas flow grooves that extend from a first edge of the rectangular bipolar separator to the opposite edge. The oxidant-contacting side has molded parallel oxidant gas flow grooves that extend from a second edge of the rectangular bipolar separator to the opposite edge, such that the fuel and oxidant grooves are perpendicular to each other. The oxidant-contacting side is coated with a thin, dense YSZ electrolyte film that is then coated with a thicker porous LSM cathode film. The thin metallic foil is compliant enough in compression to conform to the mating surfaces and provide a seal. Further, it is thin and malleable enough that it does not generate excessive stresses even with some mismatch in thermal expansion characteristics. The foil is specified as a superalloy containing chromium, with an optional nickel or copper coating on the fuel side to eliminate formation of an electrically resistive oxide film. The Virkar patents are designed for use with external manifolds, where internal manifold stacks are not disclosed. Further, anode edges are exposed to oxidant gas in the oxidant manifolds, and cathode edges are exposed to fuel gases in the fuel manifolds, which could lead to undesirable reactions between fuel gas and oxidant gas, as well as degradation of the electrodes. Similarly, nickel or copper on the fuel side of the bipolar separator may react with oxidant gas at the exposed edges in the oxidant manifolds.
In conclusion, the prior art does not describe SOFC cell stacks, particularly internal manifold stacks, that take into account technical and cost considerations in order to produce durable, economically competitive fuel cell power generation systems.